Coating compositions including pigment systems are well known and widely used in a variety of applications on different substrates. The coating compositions may be of a wide variety of types and are commonly characterized by having a pigment system including titanium dioxide as a pigment component for producing desired opacity or hiding in the resulting coating.
Generally, titanium dioxide is considered essential within such pigment systems because of its uniquely high index of refraction. The selection of titanium dioxide for this purpose is well known. Furthermore, it is also well known to preferably employ titanium dioxide in a rutile form having a median particle size of about 0.25 microns for producing optimum light scattering within a resulting coating.
As noted above, the general characteristics of such coating compositions including components other than the pigment system and the selection of titanium dioxide within the pigment system is well known. Accordingly, no further discussion of the desirability for employing titanium dioxide in such pigment systems is set forth for purposes of the present invention. However, it is to be understood that the present invention relates to coating compositions having titanium dioxide alone or usually in combination with other pigment components in their pigment systems, the titanium dioxide preferably being of a rutile form and having a median particle size of about 0.2 microns to about 0.3 microns, preferably about 0.25 microns, as noted above.
Pigment volume concentration (hereinafter referred to as "PVC") is a particularly important and well known parameter in connection with such coating compositions and particularly their pigment systems. That term is also important in further defining the field of the present invention in accordance with the following discussion.
With a PVC or titanium dioxide content of less than about 21-22%, titanium dioxide generally functions in an efficient manner to produce opacity as defined by hiding power. For purposes of the present invention it is to be understood that opacity and hiding are generally equivalent. Thus, in that range, hiding power is generally proportional to the amount of titanium dioxide in the pigment system. In most cases, it is generally necessary or desirable that titanium dioxide constitute a substantial portion of the pigment system in that range, at least for developing maximum hiding. Generally, any dilution of the titanium dioxide with other extender or filler pigments merely results in a reduction of hiding corresponding to the amount of titanium dioxide replaced by the other or substitute pigments, herein referred to as filler pigments.
An opposite extreme occurs in coatings produced from such coating compositions when the coating composition has a pigment system which is above a so-called critical pigment volume concentration (hereinafter termed "CPVC"). This term is also well know in the field of coatings and generally indicates a point at which the resin component in the coating formulation is no longer sufficient to entirely coat all of the pigment particles in a resulting coating.
Accordingly, for coating compositions above the CPVC, it is possible to rely on "dry hiding" where the index of refraction for titanium dioxide is no longer as critical. Generally, in such high PVC concentrations, other pigments which are very low cost in comparison to titanium dioxide, and which typically have much larger particle sizes than that referred to above, may be employed since the pigment particles are not entirely coated by the resin component in a resulting coating and the index of refraction for the pigments is less critical.
Actually, "dry hiding" begins to exhibit an effect well below the CPVC, usually at about 50% PVC, and becomes the dominant factor as soon as the CPVC is exceeded. Titanium dioxide is required for "wet hide"; otherwise, the coating or paint would have a very low level of opacity when wet.
The effect of a pigment in producing opacity is considered as follows. Light scattering occurs at interfaces between the resin and pigment particles and also between air and pigment particles. The greater the difference in refractive index for materials at the interface, the greater the amount of light scattering. For example, air has a refractive index of 1.0, most resins about 1.5-1.6, inert extenders or fillers about 1.55-1.65, rutile titanium dioxide about 2.71 and anatase titanium dioxide about 2.55. The indices of refraction for air and extenders are thus far enough apart to generate sufficient light scattering to develop hiding. Resins and extenders have about the same refractive index so that the extender is generally transparent within the resin. Resins and titanium dioxide are sufficiently far apart in refractive index to produce good hiding, thus the preference for titanium dioxide as a white hiding pigment.
The considerations discussed above for pigment systems either below a PVC of about 21-22% or above the CPVC are well known to those skilled in the art. It is also well known to those skilled in the art that in an intermediate range between a PVC of about 21-22% and up toward the CPVC, titanium dioxide tends to exhibit so-called "crowding" in a resulting coating so that maximum hiding efficiency is generally not achieved at least when titanium dioxide is employed along within such a pigment system. For this reason, substantial effort has been expended in the past to develop suitable spacer or extender pigments which are of relatively low cost, compared to titanium dioxide, in an attempt to minimize the crowding effects and optimize hiding efficiency of the titanium dioxide component in the pigment system. Generally, as noted above, such spacer or extender pigments have been most successful in pigment systems approaching or exceeding the CPVC where dry hiding power of the additional pigments can be employed.
For purposes of the present invention, coating compositions are contemplated as including a prime pigment or pigments having the primary functions of providing color and opacity in the resulting coating or paint. At noted above, titanium dioxide is commonly employed as the prime pigment for white compositions and also for substantially white formulations including tints of white or pastel colors. Other color pigments are employed with titanium dioxide in the pastel shades with lesser amounts of titanium dioxide being employed to produce deeper colors. Otherwise, the selection of such pigments is well known to those skilled in the art.
At the same time, other pigments which are commonly much less expensive than titanium dioxide are employed for other functions such as controlling gloss, controlling surface texture, and affecting exterior durability, porosity, permeability, etc. Generally, as noted above, such filler pigments are relatively large, typically in the range of 2-40 microns.
Accordingly, the present invention is defined below in connection with coating compositions having a prime hiding component and optionally a filler component. As further defined for the invention below, the coating compositions necessarily include at least about 21-22% by volume of titanium dioxide in order to exhibit "crowding". Furthermore, because of the substantial amount of titanium dioxide necessarily present according to the present invention, the coating compositions are also substantially white, including white formulations with titanium dioxide alone and tints of white or pastels where titanium dioxide is present above the range of 21-22% by volume of the total solid content of the coating.
The functions of crowding and spacing in coating compositions including titanium dioxide pigments, at least in accordance with the prior art for the present invention, was possibly best explained and defined by Juergen H. Braun, in an article entitled "Crowding and Spacing of Titanium Dioxide Pigments", Journal of Coatings Technology, Vol. 60, No. 758, March 1988, pp. 67-70.